The History of Biological Theories

Rauber's work received little attention; similar ideas, however, gradually became more widespread. With the development of 'Evolutionary Mechanics', an endeavour to arrive at an experimental explanation of the shape of the animal body, the attention of the scientist was directed once more to animal form, and the cell theory which pictured the body as a mere aggregate of cells lost its predominance. Roux, who began this type of work, and Driesch both pointed out that too much importance had been attached to the cell theory. Whitmann (1893) made a list of the ob- jections to it. He showed that we lose sight of the unity revealed in each organism when we emphasize the in- dividuality of its cells. He said that the sort of systematic zoology which is based upon the segmentation of the

egg-cell and the results of that segmentation is unnatural, since one and the same organ (e.g. the organs of excretion), may be one-celled or many-celled. The cellular nature of such organs cannot, then, be a matter of fundamental importance. In 1894 Verworn published his General Physiology, con- taining a belated attempt to introduce the cell theory, as expressed by Schleiden, Virchow, and Haeckel, into physiology. Physiologists gave the book a cool reception. Nevertheless, even at the present day it is often con- sidered that the problems relating to the cell, to proto- plasm, to the nucleus, and to the chromosomes are the most fundamental biological problems.

FROM the earliest times heredity has been thought of as a fate which relentlessly pursues mankind. The state of being 'born in sin' of Christian philosophy ; the revenge of the God of the Old Testament 'unto the third and fourth generation'; 'Fate' which drove the heroes of Aeschylus to destruction; St. Augustine's doctrine of predestination, each man condemned to eternal happiness or eternal damnation these all contain primitive ideas about heredity ; they all suggest that the individual, although a free and conscious agent, is subject to a higher power a power which goes back to the time when that individual was not in existence. The same thought found expression in the belief in the transmigra- tion of souls, in the caste system, in . the granting of hereditary privileges.

Darwin's theory of heredity aroused considerable dis- cussion, but he was not the first to focus attention upon this subject. J. F. Meckel, the German anatomist, in a book on human anatomy which he published in 1812, gave a list of facts which point to the inheritance of monstrosities in certain individual families. Before Darwin's time anatomists as a class were not, however, interested in problems of heredity. They left such work to their medical friends. In 1847 a French doctor, Lucas, published a book in two volumes which contained a mass of material often of a very fantastic nature relating to the inheritance of mental qualities. The book was read by the general public, as well as by the medical world.

Ten years later the French psychologist, B. A. Morel, examined in great detail the problem of the inheritance of mental disturbances. He noted that certain diseases appear in one generation after another of the same family. " led him to formulate a theory of decadence. Those speculations which suggest that mankind will be eventually annihilated by disease are closely related to Morel's ideas. Buckle, in his History of English Civilization, discussed this subject of the inheritance of mental qualities, of talents, of characters both detrimental and advantageous. His final conclusion was that the subject must be left for the present; he pointed out that we must know more about non-inheritable characters before we can have any really scientific theory of heredity.

Darwin's idea of heredity differed from Morel's. The latter looked upon it as a force driving the family to a tragic end ; Darwin thought of it as a conservative element, which gave historic continuity to human life. Agassiz, Bronn, and Baer Darwin's predecessors be- lieved that every species, or at least every order, had arisen independently out of the earth, and that continuity was achieved because there was continuity in the ideas they embodied. Darwin introduced his view of heredity, and succeeded in giving a purely materialistic tone to all later speculations about the history of life on our planet.

Darwin did not distinguish between different hereditary characters. He believed in the inheritance of both bodily and mental qualities, of psychological traits as well as of anatomical features, of the broad outlines and of the minute details of character, in the inheritable nature of wounds, of acquired characters, of disease, of inborn abnormalities. He quotes all sorts of examples of these events, collected from the most diverse sources. He points out how, in one family, the colour of a lock of hair was inherited ; how an extremely hairy man had a daughter whose ears were hairy, and how her son developed a beard when he was only a year old. 1 He gives certain 'laws' of inheritance among them the following :

1 Altogether, Darwin's book On the Variation of Animals and Plants seems very old-fashioned to-day. This is the book in which he discussed heredity. All the weaknesses of his philosophy become very obvious in it. They are somewhat veiled in the Origin of Species by the abstract manner in which the problems are attacked. (a) Direct inheritance implies the passing on of qualities from parents to children. (b) One parent is prepotent, when the children derive more from that one parent than from the other.

Some writers believe that, as a rule, the female resembles the father more closely, the son the mother; others maintain the opposite view. Darwin himself was convinced that the qualities of the father pass chiefly to his sons, those of the mother to her daughters, and he explained the occurrence of secondary sexual characters in this way. (c) The influence of time. The hereditary quality appears in both parents and offspring at the same time of life. Male birds develop their characteristic colouration at maturity, because, says Darwin, they first gained these colours at this period of life through natural selection.

(d) Latent qualities. Qualities, which, though present, have not yet appeared, are called latent. Sexual desire is 'latent' in the years of childhood, song is ' latent ' in young birds, the characteristics of the frog are 'latent' in the tadpole. (e) Sometimes qualities may remain latent throughout life. The queen bee must possess the qualities, though 'latent, of the drone, for drones are produced by un- ffertilized queens. If a child shows some character which was not possessed by its parents, but in which it resembles an uncle, grandfather, grandmother, or other more distant relative, that character must have been latent in the parent.

(/) Qualities possessed by descendants, but not by their immediate ancestors, are called atavistic (atavus = great- grandfather). In practice we call characters atavistic when they suddenly appear in an individual or a race, and are such as might, according to our theories, have belonged to some very remote ancestor. The domestic pigeon, for example, is probably descended from the rock pigeon, which is grey, with two dark bands on its wings. This colour sometimes appears in domestic pigeons whose parents exhibited a very different coloura-

tion. The colour and markings are then called atavistic. The horse is believed to be descended from the wild, grey horse, which has a dark stripe down its back. This dark stripe occasionally appears in young colts, and so is called atavistic. A great deal of crossing between different races is believed to favour atavism. 1 The subject has received much attention; atavism is supposed to be exhibited even by man. Almost every monstrosity was looked upon as a 'heark back' to uncivilized man, to the monkey, or to some still more remote ancestor e.g. extreme hairiness, development of the inter- maxillary bone, were regarded as ape-like characters; six fingered extremities were even traced back to certain marine reptiles of the Permian epoch !

From many quarters there came objections to these theories. Virchow did not deny that atavism may occur, but he pointed out that some of the cases where men have resembled animals Theriomorphy may be the direct result of disturbances occurring during development. He admitted that the heart may be poorly developed, that it may even resemble, in some ways, a reptilian heart ; but this, he considered, is not a sign of atavism, but of mal- development.

During the Darwinian heyday, it was thought to be of the utmost importance to collect actual details which illustrated the passage of characters from parents to off- spring, as if heredity had first been discovered by Darwin ! Darwin collected a mass of such material in his book on The Variations of Animals and Plants under Domestication. The French psychologist Ribot, in a very uncritical book, gave examples which dealt with a rather different subject-matter.

The Swiss, de Candolle, investigated the ancestry of the members of the French Academy (1873), and as a result claimed that intellectual capacity is inheritable. Francis Galton, in England, collected a mass of statistics dealing 1 A detailed analysis of the laws of heredity, from the old-fashioned point of with heredity (1869 onwards). He maintained that size of body, skin colour, eye colour, muscular strength, in- telligence, strength of will, morality, and finally, genius, are all inherited.

Variations introduce new possibilities into life they represent a progressive element; heredity fixes is con- servative ; but if it fixes acquired characters, it works for progress. Most of the biologists who agreed with Darwin Darwin himself, Haeckel, Galton, &c., thought of heredity in this more progressive sense. Galton collected statistics to show that the inheritance of mental capacity is a factor in human progress. Darwin's doctrine, that humanity has progressed in the past and will do so in the future, was based on his conviction that spiritual, in- tellectual, and moral qualities are inherited.

Philosophers who were not biologists have tended to emphasize the aspects of heredity which make it seem more like a doom, causing undesirable qualities to be passed on from father to son, a doom pursuing the whole family to its final destruction. Particularly in connexion with mental diseases, sexual diseases, and drunkenness, the con- sequences of heredity were painted in very dark colours. Morel, to whom we have already referred, introduced this point of view. From his observations he inferred that mental disease pursues the family like a tragic fate; it passes from father to son, growing from small beginnings to a mighty destroying force, which drags the family down towards an inevitable decadence. Morel's pupils carried his theory further ; they asserted that 90 per cent, of all those who develop mental disease inherit this from their ancestry though it is not necessarily the same disease that arises.

Zola expressed this terrible view of heredity in litera- ture. Ibsen made inherited degeneracy one of his chief dramatic themes. Writers on racial questions have tended to emphasize the same point of view and often infer that the human race is deteriorating. on the wane. Statistical methods are being used, and they give results that are by no means uniform; by the use of such methods there creep in certain special errors. Statistics on these matters were usually collected in asylums. If the relations of the patient had suffered from mental disease, this was taken to be a proof of inheritance ; if they were free of it, a proof of non-inheritance. If we investigated poverty in the same way, taking a large number of the poor, and determining statistically in how many cases their poverty was inherited, we should get a very black view of life ; but it would be an incorrect view, because it would neglect all those cases where poverty was successfully overcome. So statistics culled from asylums do not take into account the cases where mentally de- ranged people have healthy children for these do not come into touch with such institutions.

Many statisticians have pointed out this fault, and, using more critical methods, have obtained less depressing results. Diem (1905), for example, is led to the following conclusions from his observations: (a) If we consider the total heredity (i.e. the inheritance of all those diseases believed to be passed on from one generation to another), the burden of the sane is little less than that of the insane. In other words, the difference in health between the two sets of ancestors is hardly noticeable.

() Among the ancestors of the mentally diseased there were, however, four times as many cases of mental disease, as among the ancestors of the mentally healthy. There were also more cases of drunkenness and of certain abnormalities of character. Both sets of ancestors show about the same tendency to nervous diseases and to suicide. On the other hand, the hereditary burden of the mentally unsound is two to four times less, in regard to apoplexy and senile dementia.

(c) We obtain different results again, if we consider the hereditary burden due to parents only as contrasted with that due to other relations. Characters are either inherited or acquired ; the organ- ism brings inherited characters into the world with him e.g. the ability to speak; acquired ones come to him during his lifetime from the outside world e.g. the knowledge of any definite language. Recently, inherited characters have been classified; Delage, for example, differentiates between anatomical (e.g. a birthmark), physiological (e.g. duration of life), psychological (e.g. character), pathological (e.g. mental disease), teratological (e.g. supernumerary fingers), and latent characters. 1

It is important to note that Darwin, in the Origin of Species, does not emphasize the difference between in- herited and acquired characters. Among characters that are acquired Darwin includes those related to use and disuse (probably under the influence of Lamarck's views) ; but the two things are not quite identical. Seidlitz and Nageli seem to have been the first to note the difference. Nageli, in 1865, pointed out the lesson to be deduced from his experiments with Alpine plants. Transplanted to the lowlands, they lost in the first generation the properties acquired as the result of their Alpine habitat. Thus the acquired characters were not inherited, (in the same year Seidlitz, a young follower of Darwin, in criti- cizing Haeckel suggested that it is doubtful whether ac- quired characters are inherited, and that to believe that they are is to revive Lamarckism)

^.Weismann's theoretical work owed much to Nageli. Still under the influence of an earlier idealistic mor- phology, Nageli believed internal organization to be the important factor in the life of the organism, while Darwin attached great importance to its history. Nageli, who was a pure materialist, pictured inner organization as due to a definite arrangement of invisible particles in the pro- toplasm. ^) Weismann began by accepting this view, and com- bined with it the idea that acquired characters are not

inherited. He wrote a series of articles in defence of this view, and met with very severe criticism. Herbert Spencer and Romanes strongly upheld the doctrine of the inheri- tance of acquired characters. Romanes did this as a true follower of Darwin, Herbert Spencer because his whole philosophical system depended on the truth of the doctrine. Human advance, according to him, depends on the inheri- tance of acquired moral and intellectual ideals. Wallace, starting from other premises, favoured Weismann's view. He believed that natural selection is practically the sole cause of evolution, and so could afford to neglect this very doubtful question of the inheritance of acquired characters.

Both parties quoted facts which seemed to confirm their views. Cases were cited of cats with short tails who had given birth to similarly deformed kittens. On the other hand, it was pointed out that, among the Jews, in spite of thousands of years of circumcision, this injury is not inherited by their descendants. The other side then pointed to Broca's measurements of the human skull. He had asserted that the average capacity of the human skull has increased, as the result of mental exercise, from 1,409 c.cm. in the twelfth century to 1,442 c.cm. to-day. So the controversy continued !

Brown-Sequard's experiments roused much excitement. He injured the brain of a guinea-pig, causing loss of sensation in the toes; by injury to another part of the brain he brought about a clouding of the transparent sub- stance in a part of the eye. The animal bit off the injured toes, and Brown-Sequard asserted that both amputated toes and cloudiness of eye were transmitted to the off- spring. The experiments were not confirmed, and the belief in the transmission of injuries is now universally abandoned.

The subject of the hereditary nature of certain diseases e.g. tuberculosis, syphilis, &c., roused much interest. In some of these e.g. mental diseases the victims are born with a disposition to the disease. In other cases we must distinguish between an inherited disposition to disease and the disease itself, which is not inherited but conveyed by a definite infective organism. What is inherited in tuber- culosis is a certain bodily weakness, a predisposition ; the disease itself is caused by infection from the environment. The disease bacteria might infect the egg cell or the spermatozoon ; they might be transmitted via the uterus to the child. Such a process is not inheritance.

In more recent times, when the Darwinian theory had lost some of its original importance, the belief in the in- heritance of acquired characters was revived. Upholders of this view asserted that they could now cite definite facts in its support these facts, however, appear much less important than their upholders would have us believe. Nageli had said that structural modifications due to climatic changes are not inherited; the Norwegian botanist Schiibeler now (1862) claimed to have proved that they are, but his observations were not confirmed by other workers. By exposing certain butterfly pupae (Vanessa) to an unaccustomed temperature, Standfusz (1905) succeeded in altering the markings of the butter- flies into which the pupae developed. In one case this new marking was inherited, although both caterpillar and pupa developed at the normal temperature. Other writers, using similar methods, obtained even better results. C. Schroder (1903) found that changes in butter- fly markings caused in this manner could be passed on through three generations. E. Fischer (1902) obtained similar results. A. Pictet (1902) succeeded in producing inheritable changes in the markings of caterpillar and butterfly by changing the food of the caterpillar.

Experiments on the immunity of animals to disease seem to point to the fact that this can be inherited in certain cases. Thus, Tizzoni and Cattaneo (1892) assert that mice immune against tetanus, and rabbits immune against rabies, pass on this immunity to their offspring. Behring makes similar assertions about diphtheria, Ehrlich (1891) in regard to the poisons abrin and ricin. inheritance of acquired characters are quoted. Goebel reports that Micrococcus prodigiosus, a bacterium which forms red patches when grown on an organic substration, loses this power if cultivated on an artificial medium ; and this loss of the power to produce the normal red colour lasts for several generations, even when these are cultivated once more on potato. W. Jennings, a critical American observer, has written a very exhaustive account of the inheritance among the protozoa of artificially produced characters (1920).

Criticism has robbed many of these experiments of their importance. For one thing the newly acquired character is not fixed, but becomes weaker in the second and third generations, and is soon completely lost. Thus its inheritance suggests the continued effect of an artificial stimulation not a change in the nature of the organism. No lasting change in constitution due to an acquired character has yet been proved. Mendel's work has given the problem of inheritance a different aspect, which is linked with pre-Darwinian idealism.